1. Field of Invention
This invention pertains generally to the production and use of microcapsules and more particularly to a method for producing small cross-linked microcapsules in a single step by spray drying, wherein polymer gelation occurs during spray drying upon volatilization of a base and rapid release of otherwise unavailable multivalent ions as the pH is reduced. A range of small to large microcapsules can be produced.
2. Background
Encapsulation of bioactive moieties is a common practice in the food, biotechnology and pharmaceutical industries to increase the stability and shelf life of the encapsulated compound and to control its delivery. In general, the encapsulation matrix confers a protective layer against adverse environmental conditions and regulates the release of the encapsulated compound in the target application.
Polymers are typically used as the encapsulating medium which allows cross-linking between the molecules to improve overall stability of the encapsulated product. One example is the use of a charged polymer as the encapsulation matrix such that multiple polymers are cross-linked via electrostatic interactions with multivalent ions. This form of ion-mediated cross-linking occurs spontaneously upon contact between polymer and ions, and rapidly converts a low-viscosity solution to a gelled mass.
Among encapsulation materials, alginates are preferred because of being non-toxic, biocompatible and relatively inexpensive. Alginic acids (alginates) are negatively charged polysaccharides readily cross-linked by divalent calcium ions and ubiquitously utilized in biotechnology and food applications. Chemically, alginates are linear copolymers of [1→4] linked β-D-mannuronic acid (M) and α-L-guluronic acid (G), arranged as blocks of either type or as a random distribution of each type. They are generally obtained from marine brown algae and have varied chemical structure and composition depending on the source and harvesting season. An important property of alginates is that they can selectively bind multivalent cations (e.g. Ca2+, Ba2+, Zn2+, and Al3+) in a gentle and almost temperature independent manner. This gentle solution to gel transition in the presence of selected cations makes alginates an ideal immobilization matrix.
One conventional encapsulation method of forming cross-linked alginate beads involves dissolving or dispersing the bioactive compound, cells or chemical in an alginate solution and promoting cross-linking by dispersing it into a solution containing the cross-linking agent, known as the diffusion setting or external gelation method. However, direct mixing of alginate and multivalent cations rarely produces homogeneous gels due to the very rapid binding kinetics of such ions. The result is a gel or beads with the highest cross-linked alginate concentrations at the outer surface with a decreasing gradient of cross-linking towards the center of the gel. A different approach known as internal gelation mixes alginates with a cross-linking agent (generally Ca2+) in a complexed or unavailable form and the cation becomes available as the pH changes. This method is generally accompanied by emulsion and vigorously stirring, or by introducing the cross-linking agent using a crystal gun. In any case, both encapsulation methods are costly, not easily scaled-up and generally limit the particle size to ≧300 μm. Overall, current methods for producing stable alginate gels that involve dropping alginate suspensions into divalent cation solutions are difficult to scale-up and produce undesirably large alginate beads.
In contrast, spray drying is a relatively inexpensive and easily scaled-up technique that is reproducible and one of the most commonly used encapsulation methods in industrial settings. The traditional spray encapsulation process involves dissolving or dispersing the active agent in a sodium alginate solution, forcing the solution through an orifice to form a droplet which is then cross-linked by contact with a calcium chloride solution. Effective spray-drying relies on pumping a low-viscosity solution through an atomizer which has historically precluded ion-mediated cross-linking.
State of art methods for encapsulating biological molecules, cells and chemicals in cross-linked alginates include variations on methods to extrude droplets of alginate/target specie solution into a calcium solution and are limited in the size of the produced particles such that only large (millimeter range) diameters can be achieved. The formation of small (micron-scale), stable particles by spray drying has not been practical due to rapid gelation of alginate upon contact with divalent cations. This process has been limited to producing particles larger than 500 μm.
Microcapsules can contain many different types of materials and can be used for both therapeutic and non-therapeutic applications. In therapeutic applications, the size of the microcapsules can be an important factor in the delivery of the capsules across cell membranes as well as the response made by the cell to the microcapsules. It has been shown that smaller microcapsules approximately 300 μm or less tend to avoid a significant cellular inflammatory or immune response and can efficiently cross membranes compared with larger microcapsules. There are many other uses for microcapsules that are smaller than the 500 μm limits of traditional spray drying methods in a wide variety of applications.
Accordingly, there is a need for methods for efficiently producing small microcapsules with reproducible characteristics that is inexpensive and can be scaled up for industrial applications. The present invention satisfies these needs as well as others and is generally an improvement over the art.